High pressure studies of configuration interaction and crystal field effects in Sm2+

Full text: Divalent rare earth ions are interesting luminescence centres because of the low energy of the excited 4fn-15d1 configuration relative to the 4fn ground configuration. The low energy difference between these two configurations leads to two principle effects which distinguish the luminescence properties of divalent rare earth ions from those of trivalent rare earth ions. First, a significant amount of 5d state mixing into the electronic states of the 4fn configuration occurs and second, the thermal activation barrier to 4fn → 4fn-15d1 crossing is greatly reduced. The first effect introduces opposite parity character into the emitting levels of divalent rare earth ions and acts to shorten lifetimes and increase f-f luminescence intensity, while the second effect acts to enhance thermal quenching of 4fn excited electronic states closest in energy to the 4fn-15d1 configuration. The interaction between the 4fn and 4fn-15d1 configurations and crystal field properties are typically studied by considering the luminescence properties of divalent rare earth ions in a series of host crystals. We are currently developing a new approach, based on high pressure luminescence spectroscopy, for understanding con-figuration interaction and crystal field properties of divalent rare earth ions. The strategy of our approach is to use high pressure as a tool of structural perturbation. By applying hydrostatic pressure to solids, we have an opportunity to continuously vary the nearest neighbour coordination environment of divalent rare earth dopants. Our general goal is to correlate pressure-induced changes in local structure with pressure-induced changes in luminescence properties in an attempt to better understand structure-property-composition relations in solid state luminescent materials. In this paper we present recent results on Sm2+in a series of MFCl (M = Sr, Ba, Ca) host lattices. Luminescence spectra and decay properties as a function of pressure up to 200 kbar will be presented. The primary effect of pressure on the electronic structure of Sm2+ is to decrease the energy difference between the 4fn and 4fn-15d1 configurations. At ambient pressure, the 4fn-15d1configuration is higher in energy than the 5D2 level of Sm2+and, depending on the temperature, it is possible to observe emission from the 5D2,1,0 levels to several of the 7FJ ground levels. Upon application of pressure, the 4fn-15d1configuration shifts to lower energy and successively crosses the 5D2 and 5D1 levels of Sm2+. This leads to a quenching of luminescence from these levels and enhancement of luminescence from the 5D0 level. At very high pressures, we begin to see evidence of a new fast decay component in our lifetime measurements. We believe we are approaching the 5D0-4fn-15d1crossover and beginning to stabilize f-d emission. We present an analysis of our results in terms of crystal field, configuration coordinate and luminescence dynamics modelling